Field of the Invention
The present invention concerns a method in order to acquire magnetic resonance (MR) data (for example to create MR images) and a method in order to determine a B1 magnetic field. Moreover, the present invention concerns a magnetic resonance system that is designed in order to implement one or both of such methods.
Description of the Prior Art
Precise knowledge of the B1 magnetic field, which is the strong, static field that is present in the examined region of the examination subject, is of great importance for many applications in the field of magnetic resonance tomography, for example for the calculation of an RF pulse in multichannel transmission operation or for quantitative T1 examinations. In a strong, static B0 magnetic field (for example of a strength of 3 Tesla or more), distinctive spatially dependent variations of the B1 magnetic field can occur due to subject-specific variations in conductivity and susceptibility. Therefore, a subject-specific determination of the actual B1 magnetic field that is present is indispensable for many applications given a defined, set transmission power.
One possibility to determine the actual B1 magnetic field is offered by what is known as the DREAM method (“Dual Refocusing Echo Acquisition Mode”) as described in “DREAM—A Novel Approach for Robust, Ultra-Fast, Multi Slice B1 Mapping”, K. Nehrke et al., Magn. Reson. Med. 20 (2012): Page 605. As will be explained in more detail in the following, the B1 amplitude of the B1 magnetic field generated by resonant RF pulses is determined in DREAM depending on intensities of a stimulated echo and a gradient echo.
In other known MR methods with which MR images are generated, a stimulated echo is generated that is detected upon readout of the MR data. A problem can occur that a magnetization prepared in a previous sequence negatively affects or adulterates the MR data acquired for a current sequence. For example, artifacts can occur in an MR image reconstructed from the MR data. If such an MR method based on the stimulated echo is used to determine the B1 magnetic field, in particular the intensity of the gradient echo is disadvantageously adulterated (more precisely, increased) due to this problem, such that the determination of the B1 magnetic field is also adulterated.